I. Field of the Invention
This invention relates to microcell electrochemical devices and assemblies, and method of making the same. More specifically, this invention relates to microcell electrochemical devices and assemblies that comprise current collectors of high electrical conductivity, high corrosion resistance, and high mechanical endurance and resistance to peeling and wearing.
II. Description of the Art
Electrochemical cells, such as batteries and fuel cells, have positive and negative electrodes that are separated in such manner as to avoid internal short-circuiting, while the electrodes are in contact with an electrolyte medium, which chemically reacts with the electrodes to generate flow of electrons, thus providing electrical energy when the circuit is coupled with an external load.
A recent innovation in the electrochemical energy field is the development of microcells—small-sized electrochemical cells for battery, fuel cell and other electrochemical device applications. The microcell technology is described in U.S. Pat. Nos. 5,916,514; 5,928,808; 5,989,300; 6,004,691; 6,338,913; and 6,399,232, all to Ray R. Eshraghi. The microcell structure described in these patents comprises hollow fiber structures with which electrochemical cell components are associated.
The aforementioned Eshraghi patents describe an electrochemical cell structure in microfiber form, which contains a first current collector, a membrane separator, electrolyte medium, and a second current collector. Specifically, the Eshraghi patents disclose a hollow fibrous membrane separator, impregnated with electrolyte medium and having a first metal fiber (inner current collector) disposed inside thereof and a second metal fiber (outer current collector) disposed on the outer surface of such hollow fibrous membrane separator, wherein the first and second metal fibers constitute positive and negative current collectors of the microcell.
Multiple fibrous microcells as described hereinabove can then be serially and/or parallelly connected together, and packed/bundled into a unitary microcell assembly, which is characterized by high current density and high voltage output.
The positive and negative current collectors of the Eshraghi microcells, as described in the above-listed patents, are in contact with the reactive electrolyte medium. They therefore are subjected to a harsh chemical environment. Since the current collectors of the microcells are formed of very thin metal fibers, they are especially susceptible to corrosion, which may result in electrical disconnection within individual microcells or between serially or parallelly connected microcells, eventually shortening the useful life and/or reducing the power density of the microcell assemblies.
Eshraghi U.S. Pat. No. 6,338,913 and No. 6,399,232 suggest several corrosion management structures for enhancing the corrosion resistance of the current collectors in the microcell devices or structures. For example, current collectors can be manufactured from graphite materials; alternatively, current collectors can be formed of aluminum or copper coated with corrosion-resistant carbonaceous coatings. Although the graphite or carbon-coated current collectors have significantly enhanced corrosion resistance in comparison with the conventional metal current collectors, they are not mechanically robust (i.e., the graphite current collectors are easily broken, and the carbon coating are easily peeled off, exposing the underlying metal core to the corrosive electrolyte), which will result in eventual electrical disconnection within or between the microcells.
There therefore is a continuing need in the art to identify and utilize improved materials for forming current collectors of high corrosion-resistance, high electrical conductivity, and high mechanical integrity in such microcell structures, and it is accordingly an object of the present invention to provide a microcell structure or assembly comprising current collectors formed of corrosion-resistant materials for significantly prolonging the useful life and enhancing the reliability of such microcell structure or assembly.